In multiple-user multiple-input-multiple-output (MU-MIMO) downlink transmission, a base station (node B) is provided with M transmitting antennas, and K (K≧M) users are provided with Nk receiving antennas, when channel state information at transmit side (CSIT) is available, closed loop transmission shall be adopted to increase the spectral efficiency via multiple antennas. But in narrow bandwidth applications of the feedback link, the base station cannot accurately acquire the CSIT, thus a limited feedback strategy based on quantization shall be applied in such beamforming based close-loop systems. In MIMO-OFDM systems, as the number of subcarriers in one OFDM symbol is large (usually more than 512), and the channel fading on different subcarriers are not equal to each other exactly, the amount of feedback for beamforming over all data subcarriers are enormous, which makes the close-loop transmission impossible.
One possible scheme to the above is to divide the subcarriers in time-frequency grid into several blocks, in which the same beamforming vector is applied to all the subcarriers belonging to the same block, i.e., subchannel. The possibility of this method depends on the following two factors. Firstly, the channel fading fluctuation in time-frequency grid is directly influenced by the time and frequency selective fading characteristics, and fortunately, in most of the practical applications, the fluctuation over one subchannel is very slight, therefore the channel fading over one subchannel can be regarded as approximately flat. Secondly, the common codebook at both the mobile terminal MS and the base station B is discrete as it just consists of 2Nb precoding vectors, wherein Nb is bit number for characterizing the whole codebook. Even though the channel fading on the subcarriers belonging to the same subchannel has slight fluctuation, the selected precoders may be still suitable for all the subcarriers in this subchannel.
The two points above make the block-wise precoding scheme practical in real systems with slowly time-varying fading, and each subchannel in time-frequency grid shall consist of the subcarriers with similar fading as much as possible, thus the structure of the block shall be deliberately designed. Presently, the structure of subchannel of the downlink for transmitting data from the base station B to the mobile terminal MS is fixed.